Basically you need to follow these instructions which can found at the CS2D FAQ page.
"I want to host a game. What's my IP?
Click here to get your IP!
Other players can join with the "Connect to IP" button using your ip.
Is there a dedicated server software for CS2D?
Yes! There is dedicated server software for both: Windows and Linux servers!
You can find and download them at the download-page!"